Actually that doesn't always work...
I have a legal copy, registered with BNET and have been playing it for years on 3 different Macs so far. Latest being a unibody MBP.
It happens so often now that I don't even bother trying to quit normally. And since it has other bugs while playing online, such as the error message that another application is using the port, or suddenly it can't find your custum maps anymore, or it can't verify your version anymore. I find that the quickest way to reboot the game while playing is:
as soon as you get the error message, or want to quit playing:
CMD-M // Window mode
in a terminal window type:
ps -Ac | grep III // list active processes and only displays those with III (ie Warcraft III)
kill Number // Number = number associated with the process
then reopen the game with spotlight.
in all you can do this without even touching the mouse which makes it much faster.
And no matter what ALWAYS quit WarIII in window mode...or you may get a nasty surprise.